permissions: derive legacy exec policies at boundaries (#19737)

## Why

After config and requirements store canonical profiles, exec requests
should not cache a derived `SandboxPolicy`. The cached legacy value can
drift from the richer profile state, and most execution paths already
have the filesystem and network runtime policies they need.

## What Changed

- Removes `sandbox_policy` from `codex_sandboxing::SandboxExecRequest`
and `codex_core::sandboxing::ExecRequest`.
- Adds an on-demand `ExecRequest::compatibility_sandbox_policy()` helper
for the Windows and legacy call sites that still need a `SandboxPolicy`
projection.
- Updates Windows filesystem override setup and unified exec policy
serialization to derive that compatibility policy at the boundary.
- Updates Unix escalation reruns and direct shell requests to
reconstruct exec requests from `PermissionProfile` plus runtime
filesystem/network policy, without carrying a cached legacy policy.
- Adjusts sandboxing manager tests to assert the effective profile
rather than the removed legacy field.

## Verification

- `cargo check -p codex-config -p codex-core -p codex-sandboxing -p
codex-app-server -p codex-cli -p codex-tui`
- `cargo test -p codex-sandboxing manager`
- `cargo test -p codex-core
exec_server_params_use_env_policy_overlay_contract`
- `cargo test -p codex-core unix_escalation`
- `cargo test -p codex-core exec::tests`
- `cargo test -p codex-core sandboxing::tests`

codex-rs/core/src/unified_exec/process_manager.rs

@@ -664,7 +664,8 @@ impl UnifiedExecProcessManager {
 
         #[cfg(target_os = "windows")]
         if request.sandbox == codex_sandboxing::SandboxType::WindowsRestrictedToken {
-            let policy_json = serde_json::to_string(&request.sandbox_policy).map_err(|err| {
+            let sandbox_policy = request.compatibility_sandbox_policy();
+            let policy_json = serde_json::to_string(&sandbox_policy).map_err(|err| {
                 UnifiedExecError::create_process(format!(
                     "failed to serialize Windows sandbox policy: {err}"
                 ))

codex-rs/core/src/unified_exec/process_manager_tests.rs

@@ -110,7 +110,6 @@ fn exec_server_params_use_env_policy_overlay_contract() {
         windows_sandbox_level: codex_protocol::config_types::WindowsSandboxLevel::Disabled,
         windows_sandbox_private_desktop: false,
         permission_profile,
-        sandbox_policy,
         file_system_sandbox_policy,
         network_sandbox_policy,
         windows_sandbox_filesystem_overrides: None,